This invention relates in general to plasma arc cutting torches and their method of operation. More specifically, it relates to an improved nozzle and related method of operation particularly useful in high definition torches.
Conventional plasma arc cutting torches produce a transferred plasma jet with current density that is typically in the range of 20,000 to 40,000 amperes/in.sup.2. High definition torches are characterized by narrower jets with much higher current densities, typically about 60,000 amperes in.sup.2. High definition torches are desirable since they produce a narrow cut kerf and a square cut angle. They also have a thinner heat affected zone and are more effective in producing a dross free cut and blowing away molten metal.
One problem with high definition torches is the need to cool the nozzle very efficiently. A cold nozzle wall at the nozzle exit orifice produces a thin boundary layer of cooled gas. This protects the nozzle and pinches the arc, that is, causes the arc to contract by this energy drain from its boundary. In order for the plasma arc to maintain enough charge carriers (ionization) to continue to conduct electrical current, the plasma arc will react to the constricting by increasing its centerline temperature and thus its ionization rate. This cooling is also important to control double arcing and gouging of the nozzle orifice, which occurs when the arc contacts the wall. Water cooling has proven effective in achieving the necessary degree of cooling.
Another problem is that even though the gas pressure and density in the plasma chamber are more than twice the values of these parameters in conventional torches, the mass flow rate is still lower than that in conventional torches. This is caused by the necessarily small diameter nozzle orifice of a high definition nozzle, which quickly chokes the plasma gas, yeilding a very low mass flow rate. These conditions choke the flow of plasma gas out of the torch. The low mass flow rate of the choked flow produces at least three other problems.
First, the low mass flow rate destabilizes the point of attachment of the arc on the electrode, or more precisely, on the hafnium or zirconium insert at the lower end of the electrode. As a result, the insert exhibits a severe random pitting rather than an even, parabolic wear pattern. The life of the electrode is therefore reduced. Also, as the point of attachment shifts over the insert, the entire arc column destabilizes. Destabilization from any source greatly reduces cut quality; the cut angle is worse and dross builds up under the workpiece. Second, the low mass flow rate means that there is a weak cold flow around the arc as it interacts with the nozzle. With a thin cold wall, the shape of the arc is very sensitive to the shape (condition) of the nozzle. As a result, cut quality becomes very sensitive to the condition of the nozzle. A slight nick in the edge of the nozzle can significantly alter the cut angle and produce dross. Third, the time required to eject a pilot arc out of the nozzle, as required for arc transfer, is greatly increased by a low flow rate associated with high definition torches. Copper oxide has a tendency to build up on the inner surface of the nozzle during the pilot arc state. The nozzle surface becomes very rough after several hundred starts. The nozzle life is thus reduced.
In the past, various types of plasma gas flows have been used to stabilize the arc. One technique, discussed briefly above, is cold wall stabilization. The wall of the nozzle adjacent the plasma jet is cooled. This creates a boundary layer of cooled plasma gas that contracts the arc and holds it spaced from the nozzle. Another technique is vortex flow stabilization. Plasma gas is injected into the plasma chamber tangentially to impart a swirling movement to the gas as it flows axially through the plasma chamber toward the nozzle exit orifice. As the gas rotates in the chamber, hotter, lighter gases remain near the center, while cooler, heavier gases are driven by centrifugal force toward the outer walls of the chamber. This produces a cool gas boundary layer that stabilizes the arc. Finally, sheath stabilization has also been used. A large axial flow of comparatively cold gas surrounds the arc. Heat from the arc dissipates to this cold sheath, causing the arc to constrict, the "thermal pinch" effect, thereby stabilizing the arc.
Combinations of these techniques are also used. For example, in known high definition torches the plasma flow is typically swirled and the nozzle is water cooled. However, the mass flow rates in high definition torches are so low that the level of stabilization provided by vortex flow or cold wall stabilization are not sufficient to overcome the arc stability problems noted above. Nor do any of these prior art techniques solve the other low mass flow rate problems of high definition torches such as the accumulation of "black" (copper oxide) on the nozzle. Moreover, the straight forward expedient of increasing the gas flow to the torch does not work because of the severe choking effect of the heated gas in the plasma chamber.
It is therefore a principal object of the present invention to provide an improved nozzle and method of operation for a plasma arc cutting torch that greatly enhances arc stability, cut quality and the useful life of both the electrode and the nozzle.
Another principal object of this invention is to provide a high level of cut quality throughout the useful life of the electrode and the nozzle.
A further object is to facilitate initiation of a pilot arc using high voltage, high frequency starting.
Another object is to promote even wear on the electrode and greatly reduce the accumulation of copper oxide on the nozzle.
Still another object is to produce cuts characterized by square cut angle and substantially no dross.
Yet another object is to provide an improved nozzle and method of operation that are compatible with a method of operation requiring a complete and reliable cessation of plasma gas flow on cut off of the arc current.
A further object is to provide the benefits of a high mass flow rate of plasma gas through the plasma chamber with a strong vortex action despite flow choking at the nozzle orifice.
A yet further object is to provide all of the foregoing advantages utilizing known materials and manufacturing techniques and a favorable cost of manufacture.